Vehicle suspension unit

ABSTRACT

The invention provides a diaphragm-type hydraulic displacer unit of the kind which includes a piston-like member arranged to move inwardly and outwardly of a cylinder-like member, the annular gap between the piston and cylinder being bridged by a flexible diaphragm and in which during operation the ration between the maximum stroke of which the unit is capable and the maximum effective bore of the diaphragm, occurring in operation, is at least 1.2 : 1, and may be 2 : 1 or more, and in which during all operative conditions the pressure across the unsupported diaphragm loops exceeds 100 pounds per square inch. The invention is concerned with ensuring longevity of the diaphragm in such a unit and provides that the unit shall have the following features in combination: A. THE FLARED FLANKS OF THE PISTON AND THE INNER WALL OF THE CYLINDER SKIRT ARE TAPERED IN OPPOSITE DIRECTIONS SUCH THAT OVER THE MAXIMUM STROKE PERMITTED IN OPERATION, THE EFFECTIVE BORE OF THE DIAPHRAGM INCREASES AS THE PISTON MOVES INWARDLY OF THE CYLINDER AND SUCH THAT THE DIMENSION OF THE EFFECTIVE BORE OF THE DIAPHRAGM, WHEN THE PISTON IS IN ITS INNERMOST POSITION IS BETWEEN 15 PERCENT AND 25 PERCENT GREATER THAN THE DIMENSION OF THE EFFECTIVE BORE OF THE DIAPHRAGM WHEN THE PISTON IS IN THE OUTERMOST POSITION OF ITS PERMITTED STROKE; B. THE PISTON FLANKS AND THE INNER WALL OF THE SKIRT ARE SO CONSTRUCTED THAT THE RATIO BETWEEN THE EFFECTIVE BORE OF THE DIAPHRAGM AND THE LOOP RADIUS OF THE DIAPHRAGM WILL ALWAYS BE GREATER THAN 11 : 1 THROUGHOUT THE PERMITTED STROKE, WITH THE LOOP RADIUS BEING MAINTAINED SUBSTANTIALLY CONSTANT WITHIN 10 PERCENT THROUGHOUT THE PERMITTED STROKE; C. THAT THE DIAPHRAGM IS INITIALLY MOULDED IN THE SHAPE WHICH IT WOULD BE FORCED TO ADOPT WHEN THE PISTON IS ADVANCED TO A POSITION FURTHER INTO THE CYLINDER THAN WOULD BE OCCASIONED DURING ITS NORMAL OPERATION IN THE PERMITTED STROKE; D. THE CORD REINFORCEMENT OF THE DIAPHRAGM CONSISTS OF AT LEAST TWO PLIES HAVING A BIAS ANGLE OF BETWEEN 30* AND 50* INCLUSIVE; E. THE THICKNESS OF THE DIAPHRAGM IS SUCH THAT THE RATIO BETWEEN THE LOOP RADIUS OF THE DIAPHRAGM AND THE THICKNESS OF THE DIAPHRAGM IS ALWAYS GREATER THAN 2 : 1 THROUGHOUT THE PERMITTED STROKE.

i United States Patent Moulton etal.

m st 7 3,669,225 1 June 13,1972

[54] VEHICLE SUSPENSION UNIT [72] inventors: Alexander Eric Moulton;Harold Harman,

both of Bradford-on-Avon, England [73] Assignee: Moulton DevelopmentsLimited, Bradfordon-Avon, Wiltshire, England [22] Filed: Jan. 22, 1971[21] Appl. No.: 108,760

[30] Foreign Application Priority Data Jan. 30, 1970 Great Britain..4,624/70 [52] U.S. Cl ..l88/298, 188/322 [51] Int. Cl ..F16f9/08 [58]Field of Search 188/298, 322; 74/l8.2

[5 6] References Cited UNITED STATES PATENTS 3,330,422 7/1967 Rasmussen..l88/298 FOREIGN PATENTS OR APPLICATIONS 443,927 10/1912 France..l88/298 978,479 4/1951 France ...l88/298 6,502,673 9/1965 Netherlands..188/322 Primary Examiner-Even C. Blunk Attorney -Shoemaker & Mattare[5 7] ABSTRACT The invention provides a diaphragm-type hydraulicdisplacer unit of the kind which includes a piston-like member arrangeds we qly 224, e a d of l eqsrrlike mem tion between the maximum strokeof which the unit is capable and the maximum effective bore of thediaphragm, occurring in operation, is at least 1.2 l, and may be 2: i ormore, and in which during all operative conditions the pressure acrossthe unsupported diaphragm loops exceeds 100 pounds per square in'ch.

The invention is concerned with ensuring longevity of the diaphragm insuch a unit and provides that the unit shall have the following featuresin combination:

a. the flared flanks of the piston and the inner wall of the cylinderskirt are tapered in opposite directions such that over the maximumstroke permitted in operation, the effective bore of the diaphragmincreases as the piston moves inwardly of the cylinder and such that thedimension of the effective bore of the diaphragm, when the piston is inits innermost position is between percent and percent greater than thedimension of the effective bore of the diaphragm when the piston is inthe outermost position of its permitted stroke;

b. the piston flanks and the inner wall of the skirt are so constructedthat the ratio between the effective bore of the diaphragm and the loopradius of the diaphragm will always be greater than 11 1 throughout thepermitted stroke, with the loop radius being maintained substantiallyconstant within 10 percent throughout the permitted stroke;

c. that the diaphragm is initially moulded in the shape which it wouldbe forced to adopt when the piston is advanced to a position furtherinto the cylinder than would be occasioned during its normal operationin the permitted stroke;

d. the cord reinforcement of the diaphragm consists of at least twoplies having a bias angle of between and inclusive;

e. the thickness of the diaphragm is such that the ratio between theloop radius of the diaphragm and the thickness of the diaphragm isalways greater than 2 1 throughout the permitted stroke, I g g 4 Claims,4 Drawing Figures PATENTEDJun 13 I972 3,669.225

sum 2 OF 2 ll lll INVE N T695 ALEX/)NOEK 52 M014 l-J'au BY HARMANATTORNEYS VEHICLE SUSPENSION UNIT This invention relates todiaphragm-type hydraulic displacer units.

There are applications in engineering, especially in vehicle suspensionsystems where it is advantageous to render fluid displacement systemshermetically sealed by the use of impermeable diaphragms.

For example, in our prior British Pat. No. 828,607, we have described adiaphragm type hydraulic displacer unit in which the diaphragm isrequired to have the ability to sustain a long stroke in relation to itseffective bore.

This invention relates to a diaphragmtype hydraulic displacer unit ofthe general type described in the aforementioned British Pat. No.828,607, an example of which is shown in FIGS. 1 to 3 of theaccompanying drawings which will now be described so that the inventionwill be more readily understood.

In the drawings, FIG. 1 is a purely diagrammatic cross-sectional sideview of a diaphragm-type displacer unit;

FIG. 2 is a detail view to greater scale than FIG. 1 illustrating thecord reinforcement applied to diaphragms of the kind in question.

FIG. 3 is a detail view to enlarged scale of the loop region of thediaphragm in cross section.

FIG. 4 shows an enlarged view of the diaphragm in various positions.

In FIG. 1, 1 generally represents a composite member being an analogueof a cylinder and 2 represents a piston-like member having outwardlyflared flanks and 3 generally represents a flexible diaphragm bridgingthe annular gap between the piston 2 and the cylinder 1.

The cylinder and the diaphragm define a space filled with liquid underpressure such that the diaphragm sustains a substantial pressuredifference across it.

The composite cylinder 1 is composed of a metal casing Ia having an openend. This open end is shaped and bent over to form a lip which trapsfirstly a metal skirt member lb which also forms part of the cylinder 1;secondly the bent over lip It at the open end of the cylinder traps theouter peripheral edge of the diaphragm generally designated 3; thirdlythe lip 1c traps a liner 4.

The diaphragm 3 has a main body or septum 3a of rubber or like materialwhich is reinforced especially in the region where it flexes inoperation with a carcase of synthetic textile cords indicated at 3b. Allthe reinforcement is embedded in the septum 3a. The diaphragm 3 isrendered impermeable by providing on its face the liner 4 which may beof butyl rubber or neoprene.

At its outer periphery the diaphragm 3 is reinforced by a textile orsteel annulus 3d to which is attached the cord reinforcement 3b. Theannulus 3d is embedded in a peripheral bead 3e which is homogeneous withthe main septum of rubber or like material.

The central region of the diaphragm is reinforced by an inner textile orsteel annulus 3f to which the reinforcement 3b is attached and thiscentral annulus 3f is also embedded in a bead 3:; which is homogeneouswith the main body or septum 3a.

The reinforcing cords 3b embedded in the septum 3a and extending fromthe outer annulus 3d to the inner annulus 3f and there are commonly atleast two plies of these cords which as shown in FIG. 2, intersect oneanother at a certain angle known as the bias angle shown on FIG. 2 as Y.If the individual cords have a diameter of say 0.010 inch, there arecommonly provided between 30 and 50 cords per linear inch.

The crown 2a of the piston 2 may be of flat shape as shown, or it may beof shallow concave, or shallow convex, shape. The crown is suitablycovered by a thin membrane forming an extension of the main body ofdiaphragm 3 and by a central zone ofthe liner 4.

The inner annulus 3f is seated in an annular groove 2b surrounding thepiston crown 2a.

The piston, in operation, is intended to move into and out of thecylinder 1 over a stroke indicated in FIG. I at S, that is to say adistance between an outermost position indicated at 51 and an innermostposition indicated at S2. During such movement the diaphragm rollsbetween the conically flared flanks 2c of the piston and the conicallytapered inner wall of the skin member lb, the conicity of the inner wallof the skirt lb being in the opposite sense to that of the piston flanks2c to an extent represented by a taper angle indicated at X.

Except for its unsupported pressure sustaining loops, the major part ofthe length of diaphragm 3 is supported by the piston flanks and theinner wall of the skirt lb throughout the permitted stroke S and due tothe opposite conical tapering of skirt 1b and the piston flanks 2c theeffective bore of the diaphragm increases as the piston 2 moves inwardlyof cylinder 1 and decreases when the piston moves in the op positedirection, without any significant change in the length of the diaphragmwhich is supported.

It is to be emphasized that the stroke S is the maximum stroke permittedto the piston, in operation. This will be limited, for example by stopsor mechanical linkages which are not illustrated, so that the extremepositions past each end of the stroke where the whole of the diaphragmwould be subjected to strain in linear tension, derived from forcesother than those occasioned by the fluid pressure across it, are neverreached or even closely approached.

The mean effective bore of the diaphragm 3 is the dimension measureddiametrally to the centers of the unsupported pressure sustaining loopsformed by the diaphragm as it rolls between the piston 2 and the skirt1b.

In FIG. 1, B represents this dimension measured diametrally between thecenters of the loops of the diaphragm 3 and for the purpose of thisspecification the maximum effective bore of the diaphragm is defined asthe maximum dimension of B, namely that reached when the piston movesinto the cylinder 1 and adopts the extreme innermost position of thepermitted stroke, S, indicated at S2.

The radius of the unsupported pressure sustaining loops is indicated atC in FIG. 3 of the drawings and extends between the center of curvatureof the loops and the midpoint of the diaphragm thickness. This radius CWill be dependent upon the shaping of the surfaces defining the annulargap between the piston and cylinder skirt.

Also in FIG. 3, D represents the thickness of the diaphragm, E thethickness of the liner and F the radius of the loop of the liner 4.

The space between the diaphragm 3 and the housing la is filled withhydraulic liquid and as the piston moves into or out of the cylinder 1,such liquid flows out of or into a port indicated at 1d formed inhousing 1a.

This invention relates, therefore, to diaphragm type displacer units ofthe kind which we have above defined with reference to FIG. 1 of theaccompanying drawings.

The invention has for its object to ensure longevity in such a displacerunit wherein during operation the ratio between the maximum stroke S ofwhich the unit is capable and the maximum effective bore B occurring inoperation is at least 1.2 1 and may be 2 1. or more and in which duringall operative conditions the pressure difierence across the unsupporteddiaphragm loops exceeds pounds per square inch.

It has been found that in order to ensure longevity in these conditionsthe construction of the diaphragm itself and of the components of thedisplacer units between which it is adapted to roll in operation,particularly so far as their shaping is concerned, requires to satisfycertain specific parameters.

According to this invention it is provided that in a diaphragm-typehydraulic displacer unit of the kind hereinabove described withreference to FIG. 1

a. firstly, the flared flanks of the piston and the inner wall of thecylinder skirt are tapered in opposite directions such that over themaximum stroke 8 permitted in operation, the effective bore B increasesas the piston moves inwardly of the cylinder and such that the dimensionof the effective bore B,

when the piston is in its innermost position is between percent andpercent greater than the dimension of B when the piston is in theoutermost position of its permitted stroke S;

b. secondly, the piston flanks and the inner wall of the skirt are soconstructed that the ratio between the bore B and the loop radius C ofthe diaphragm 3 will always be greaterthan l l 1 throughout the strokeS, with the loop radius C being maintained substantially constant with10 percent throughout the stroke S;

c. thirdly, that the diaphragm is initially moulded in the shape whichit would be forced to adopt when the piston is advanced to a positionfurther into the cylinder than would be occasioned during its normaloperation in the permitted stroke S;

d. fourthly, the cord reinforcement of the diaphragm consists of atleast two plies having a bias angle Y of between and 50 inclusive; and

e. fifthly, the thickness of the diaphragm is such that the ratiobetween the loop radius C of the diaphragm 3 and the thickness D of thediaphragm 3 is always greater than 2 1 and is preferably between 2.5 land 3 l.

Preferably the ratio between the loop radius F of the liner 4 and thethickness E of the liner is always in excess of 3.5 l and preferably 4 lor more.

Due to these parameters the strains imposed upon the liner 4 and on thereinforcement 3b of the diaphragm 3 are kept at an acceptable level. Ithas been found that displacer units constructed in accordance with theserequirements can accomplish so many cycles, over a full stroke S whichis at least 1.2 times as great as the maximum effective bore Bencountered during such stroke, and when the pressure across thediaphragm exceeds 100 pounds per square inch, that an adequate degree oflongevity is achieved.

The piston flanks and the inner wall of the skirt are preferably trueconical and oppositely tapered with the difference in taper angle Xbetweenthe flanks of the piston and the inner wall of the cylinder skirtamounting to between 7 and 13".

One embodiment of the present invention will now be described by way ofexample with reference to FIG. 4 of the accompanying drawings. I

This drawing is a transverse cross-sectional side view of the relevantparts of a long-stroke diaphragm-type hydraulic displacer showingvarious positions of the diaphragm and of the components between whichit rolls.

In FIG. 4 similar reference numerals are employed for parts analogous tothose which have already been described with reference to FIGS. 1 to 3of the drawings.

In FIG. 4, 3 again generally represents a diaphragm adapted to rollbetween oppositely tapered surfaces, firstly, of a piston 2 andsecondly, of a skirt lb forming part of the composite cylinder of anhydraulic displacer unit.

The piston 2 is adapted to move inwardly of the cylinder over a stroke Sfrom an outermost permitted position indicated at 51 to an innermostpermitted position indicated at S2. The median position of the stroke isindicated at S3. The effective bore of the diaphragm is again indicatedat B and has the dimension measured diametrally between the centers ofthe unsupported pressure sustaining loops of the diaphragm.

According to a first feature of the invention, the flared flanks of thepiston 2 and the inner wall of the cylinder skirt lb are tapered inopposite directions and are so shaped that over a maximum stroke Spermitted in operation, the effective bore B increases as the pistonmoves inwardly of the cylinder such that the dimension of bore B whenthe piston is in its innermost permitted position at 52, is between 15percent and 25 percent greater than the dimension of B when the piston12 is in the outermost position SI of its permitted stroke S.

As shown in the drawings, the flanks of piston 2 and the inner wall ofthe skirt are true conical and are oppositely tapered with thedifference in taper angle X between the flanks of piston 2 and the innerwall of the cylinder skirt lb amounting to 10. Indeed the included coneangle of the piston flanks is I 1 and the included cone angle of theinner wall of cylinder skirt 1b is 9 in the drawing. Hence in thearrangement shown the effective bore B when the piston 2 is at positionS2 is approximately 20 percent greater than the bore B when the piston 2is in position S1 of its permitted stroke S.

In the arrangement shown in FIG. 4 the stroke S is more than two timesas great as the maximum dimension of bore B, namely that achieved whenthe piston is at position S2.

We have previously identified with reference to FIG. 3 the loop radius Cof the diaphragm. According to a second feature of this invention theflanks of the piston 2 and the inner wall of the skirt 1b are soconstructed that the ratio between the bore B and the loop radius C ofthe diaphragm 3 will always be greater than 1 l 1 throughout the strokeS and it will be noted that the loop radius C will be maintainedsubstantially constant within 10 percent throughout the stroke S.

A further feature according to the invention is that the diaphragm shallbe reinforced by at least two plies of reinforcing cords indicated at 3band we have previously shown with reference to FIG. 2 that these cordsintersect one another at a bias angle Y. According to the invention theangle Y is between 30 and 50 inclusive.

We have previously defined with reference to FIG. 3 the thickness D ofthe diaphragm 3 and according to the invention the ratio between theloop radius C and the thickness D of the diaphragm is always greaterthan 2 1 throughout the stroke S. The diaphragm again has a liner 4having a thickness E and according to the invention it is preferred thatthe ratio between the loop radius F of the liner 4 and the thickness Eis always in excess of 3.5 l.

A final and essential feature of the invention is that the diaphragm 3is initially moulded in the shape which it would be forced to adopt ifthe piston 2 were advanced to a position further into the cylinder thanwould be occasioned during its normal operation in its permitted strokeS.

The moulded shape of the diaphragm is indicated at M in FIG. 4.

The reason for adopting the shape indicated at M in the moulding processwill now be briefly described.

In the manufacture of diaphragms the reinforcing cords 3b are firstconnected to the inner and outer reinforcing annuli 3d and 3f Anappropriate bias angle Y is imparted to the cords and they are thenplaced in a forming mould, the rubber or analogous material intended toconstitute the septum 3a of the diaphragm is then entered into the mouldto embed the reinforcing cords.

When the cords lie in the mould it is not practical to maintain thecords in spaced relation from sharply radiused corners of the mould.

For example, if the diaphragm were to be moulded in the shape which itadopts when the piston is at stroke position S1 the cords 3b would lienear the surface of the shaping mould at regions Oil and Q2 indicated inFIG. 4 of the drawings. It will be noted that region Qll is a region ofthe diaphragm which is unsupported and pressure sustaining during thepermitted stroke S of the piston. There is, therefore, a danger thatduring operation the fluid pressure across the diaphragm will tend tourge the rubber septum 3a away from its reinforcement 3b and that theexposed cords will abrade the liner 4.

If, however, as provided according to the invention, the diaphragm ismoulded in a position indicated at M in FIG. 4, the cords 3b will lieclose to the walls of the forming mould in the region extending from theouter annulus 3d to a point approximately indicated at P1 in FIG. 4.

The cords will also lie close to the walls of the forming mould at aposition indicated at P3 in FIG. 4. However, it will be noted thatduring the operative stroke S of the displacer unit, the regions of thediaphragm extended between the outer annulus 3d and the point P1 will besupported by the inner wall of the skirt lb while the region of thediaphragm indicated at P3 will be supported by the piston 2.

As a consequence, if the diaphragm is moulded in the position indicatedat M, at no time during its permitted stroke will regions of thediaphragm in which the reinforcing cords lie close to the surface of thediaphragm be unsupported either by the piston or by the cylinder skirtlb.

It will be appreciated that this is an important factor in ensuringlongevity of diaphragms which in practice sustain pressure which may beconsiderably in excess of lOO pounds per square inch across theirunsupported loops.

It will be noted that during operation in the permitted stroke S andparticularly at the extreme limits of stroke S, certain portions of thediaphragm and of the liner are subjected to circumferential strainsthrough having to roll between the flanks of the piston 2 and the innerwall of the skirt lb. However, if

the teaching of the invention is followed and the parameters definedabove relating to the shaping and construction of the piston flanks ofthe skirt 1b and of the diaphragm 3 are followed, the strains imposed inoperation upon the liner, upon the diaphragm and upon its reinforcementare kept at an acceptable level.

Practical tests have proved that displacer units according to theinvention have fully adequate degrees of longevity when operating at astroke S considerably in excess of the maximum dimension of bore B andwhen the pressure across the unsupported loops of the diaphragm exceeds100 pounds per square inch throughout the permitted stroke S. Indeed itwill be appreciated that in the case where the pressure across the loopssubstantially exceeds 100 pounds per square inch when the piston is inthe position S1 of its stroke S, then the pressure will be much higherand may be between 300 and 400 pounds per square inch when the piston isin the innermost position S2 of its permitted stroke S.

We claim:

1. A diaphragm-type hydraulic displacer unit which includes apiston-like member arranged to move inwardly and outwardly of acylinder-like member, the annular gap between the piston and cylinderbeing bridged by a flexible diaphragm and in which during operation theratio between the maximum stroke of which the unit is capable and themaximum effective bore of the diaphragm, occurring in operation, is atleast 1.2 l, and may be 2 l or more, and in which during all operativeconditions the pressure across the unsupported diaphragm loops exceeds100 pounds per square inch, characterized by the following features incombination:

a. the flared flanks of the piston and the inner wall of the cylinderskirt are tapered in opposite directions such that over the maximumstroke permitted in operation, the effective bore of the diaphragmincreases as the piston moves inwardly of the cylinder and such that thedimension of the effective bore of the diaphragm, when the piston is inits innermost position is between l5 percent and 25 percent greater thanthe dimension of the effective bore of the diaphragm when the piston isin the outermost position of its permitted stroke; the piston flanks andthe inner wall of the skirt are so constructed that the ratio betweenthe effective bore of the diaphragm and the loop radius of the diaphragmwill always be greater than ll 1 throughout the permitted stroke, withthe loop radius being maintained substantially constant within 10percent throughout the permitted stroke;

that the diaphragm is initially moulded in the shape which it would beforced to adopt when the piston is advanced to a position further intothe cylinder than would be occasioned during its normal operation in thepermitted stroke;

d. the cord reinforcement of the diaphragm consists of at least twoplies having a bias angle of between 30 and 50 inclusive;

e. the thickness of the diaphragm is such that the ratio between theloop radius of the diaphragm and the thickness of the diaphragm isalways greater than 2 1 through the permitted stroke.

2. A diaphragm-type hydraulic displacer unit according to claim 1,wherein the ratio between the loop radius of the diaphragm and thethickness of the diaphragm lS between 2.5

l and 3 1 throughout the permitted stroke.

3. A diaphragm-type hydraulic displacer unit according to claim 1wherein there is provided on the inner side of the diaphragm animpermeable liner whose thickness is such that the ratio between thethickness of the liner and the radius of curvature of the liner in theregion of the unsupported loops of the diaphragm is always in excess of3.5 1 throughout the permitted stroke of the piston.

4. A diaphragm-type hydraulic displacer unit according to claim 1wherein the piston flanks and the inner wall of the cylinder skirt aretrue conical and oppositely tapered with the difference in taper anglebetween the flanks of the piston and the inner wall of the cylinderskirt amounting to between 7 and 13.

1. A diaphragm-type hydraulic displacer unit which includes apiston-like member arranged to move inwardly and outwardly of acylinder-like member, the annular gap between the piston and cylinderbeing bridged by a flexible diaphragm and in which during operation theratio between the maximum stroke of which the unit is capable and themaximum effective bore of the diaphragm, occurring in operation, is atleast 1.2 : 1, and may be 2 : 1 or more, and in which during alloperative conditions the pressure across the unsupported diaphragm loopsexceeds 100 pounds per square inch, characterized by the followingfeatures in combination: a. the flared flanks of the piston and theinner wall of the cylinder skirt are tapered in opposite directions suchthat over the maximum stroke permitted in operation, the effective boreof the diaphragm increases as the piston moves inwardly of the cylinderand such that the dimension of the effective bore of the diaphragm, whenthe piston is in its innermost position is between 15 percent and 25percent greater than the dimension of the effective bore of thediaphragm when the piston is in the outermost position of its permittedstroke; b. the piston flanks and the inner wall of the skirt are soconstructed that the ratio between the effective bore of the diaphragmand the loop radius of the diaphragm will always be greater than 11 : 1throughout the permitted stroke, with the loop radius being maintainedsubstantially constant within 10 percent throughout the permittedstroke; c. that the diaphragm is initially moulded in the shape which itwould be forced to adopt when the piston is advanced to a positionfurther into the cylinder than would be occasioned during its normaloperation in the permitted stroke; d. the cord reinforcement of thediaphragm consists of at least two plies having a bias angle of between30* and 50* inclusive; e. the thickness of the diaphragm is such thatthe ratio between the loop radius of the diaphragm and the thickness ofthe diaphragm is always greater than 2 : 1 through the permitted stroke.2. A diaphragm-type hydraulic displacer unit according to claim 1,wherein the ratio between the loop radius of the diaphragm and thethickness of the diaphragm is between 2.5 : 1 and 3 : 1 throughout thepermitted stroke.
 3. A diaphragm-type hydraulic displacer unit accordingtO claim 1 wherein there is provided on the inner side of the diaphragman impermeable liner whose thickness is such that the ratio between thethickness of the liner and the radius of curvature of the liner in theregion of the unsupported loops of the diaphragm is always in excess of3.5 : 1 throughout the permitted stroke of the piston.
 4. Adiaphragm-type hydraulic displacer unit according to claim 1 wherein thepiston flanks and the inner wall of the cylinder skirt are true conicaland oppositely tapered with the difference in taper angle between theflanks of the piston and the inner wall of the cylinder skirt amountingto between 7* and 13*.